1. Field of the Invention
This invention concerns an improved nuclear steam generator having an auxiliary recirculation path in its upper shell region for facilitating the prompt and uniform mixing of wet lay-up chemicals in its water inventory.
2. Description of the Prior Art
Various techniques for mixing wet-layup chemicals in nuclear steam generators are known in the prior art. The mixing of such chemicals within the water inventory contained within the secondary sides of such generators is necessary whenever the manways of the secondary side are opened, and ambient air is allowed to flow into the interior of the generator. Such an opening of the manways is necessary from time to time so that repairmen can perform routine inspections and maintenance operations within the generator. The air that flows into the generator from the manways air contains oxygen, some of which becomes dissolved in the water present within the shell of the secondary side of the generator. If left unchecked, this dissolved oxygen can substantially accelerate the corrosion deterioration of the heat exchanger tubes contained within the secondary shell. The purpose of the wet lay-up chemicals is to remove the dissolved oxygen within the water inventory, and to render the water slightly basic in order to retard the corrosion that occurs to the tubes within the generator. The introduction and mixing of these chemicals into the water of the secondary side of the generator is often accompanied by a nitrogen sparging process, wherein pressurized nitrogen is used to displace the oxygen-containing air within the secondary shell.
In order for the wet lay-up chemicals to be effective, they must be thoroughly mixed into all portions of the water inventory of the generator. To this end, prior art mixing techniques have introduced the sparging gas through the blow-down line located at the bottom of the secondary side so that the resulting bubble agitation of the water would mix the water and chemicals. To further effect the desired mixing, recirculation pumps have been used within the secondary shell. Unfortunately, none of these techniques has succeeded in thoroughly mixing the wet lay-up chemicals with the water in the generator in a short amount of time. This is a significant shortcoming since generator down-time is very expensive. However, before one can fully understand the deficiencies of prior art mixing techniques, some basic understanding of the structure of nuclear steam generators is necessary.
Nuclear steam generators of the Westinghouse design are comprised of three principal parts, including the aforementioned secondary side, a tubesheet in which a bundle of U-shaped heat exchanger tubes are mounted, and a primary side. The primary side receives hot, radioactive water heated by the nuclear reactor. The primary side conducts this water to the inlets of the U-shaped tubes that are mounted in the tubesheet. The tubesheet and the U-shaped tubes hydraulically isolate the primary from the secondary sides of the steam generator while thermally connecting them together, so that heat from the radioactive water in the primary side is transferred to the non-radioactive water in the secondary side. The hot, radioactive water transfers its heat through the walls of the bundle of U-shaped heat exchanger tubes contained within the secondary side to non-radioactive feedwater present in the shell of the secondary side of the generator, thereby converting this feedwater into non-radioactive steam.
Structurally, the nuclear steam generator resembles a vertically oriented cylindrical shell having an enlarged portion at its upper end (see FIGS. 1A and 1B). The primary side of the generator is a bowl-shaped vessel located at the bottom portion of the shell, while the secondary side is formed from the middle and enlarged upper portion of the shell. The middle portion of the cylindrical shell contains the previously mentioned bundle of U-shaped heat exchanger tubes, while the upper shell region encloses a bank of water separators that separate water droplets entrained in the steam generated by the tube bundle. In order to uniformly recirculate the water that is removed from the steam by the steam separators, the bundle of U-shaped tubes is surrounded by a generally cylindrically shaped tube wrapper that is concentrically spaced from the shell of the secondary side of the generator. The annular space between the inner surface of the shell and the outer surface of the tube wrapper forms a downcomer path for the water droplets that collect the stream down the inner walls of the shell from the water separators. The bottom edge of the tube wrapper is spaced a short distance from the tubesheet so that the water that flows down the downcomer path will be conducted into the water that surrounds the bundle of heat exchanger tubes.
Under normal operating conditions, the water level within the secondary side of the generator is always higher than the upper edge of the tube wrapper, but lower than the upper portion of the separators contained within the upper shell region. At such a level, the water contained in the interior of the tube wrapper is free to circulate through the tube bundle, over the upper edge of the tube wrapper, through the primary separators, and down the downcomer path defined between the outer wall of the tube wrapper and the inner wall of the secondary shell. From there, the water flows downwardly until it reaches the gap between the bottom edge of the tube wrapper and the upper surface of the tubesheet, where it flows back to the bottom of the tube bundle.
Unfortunately, the aforementioned recirculation path is broken whenever the level of the water within the secondary shell is brought down to a point near or below the upper edge of the tube wrapper. Such a lowering of the water level is necessary to afford repairmen access to the upper shell region of the generator so that they can perform maintenance operations. The lowering of the water, and the consequent breaking of the recirculation path between the tube bundle and the downcomer path makes it very difficult to quickly and uniformly mix the wet lay-up chemicals into the water inventory contained within the secondary side while the maintenance operations are in progress. The time required to complete the mixing not only increases generator down-time, but also increases the amount of radiation that the repairmen are exposed to while working within the secondary side of the generator.
In order to overcome the non-uniform mixing of these chemicals within the secondary side of the generator, two different mixing techniques were developed in the prior art. The first of these techniques was the injection of the sparging gas (which was normally nitrogen) into the bottom of the secondary side of the generator through the blow-down line. The small bubbles of nitrogen served to agitate the water surrounding the tube bundle, and to effectively mix the anti-corrosion wet lay-up chemicals injected into this region of the generator. However, because the recirculation path between the interior of the tube wrapper and the downcomer path was broken, the water held within the downcomer path would not readily circulate with the water surrounding the heat exchanger tubes. The end result was that a large amount of generator down time passed before the anti-corrosive wet lay-up chemicals were uniformly mixed throughout all parts of the water inventory contained in the secondary side. The second prior art technique employed was the installation of a pump and a plurality of hoses for forcing a circulation between the water in the downcomer path and the water surrounding the tube bundle while sparging gas bubbles were admitted through the blow-down line. While this pump and bubble agitation technique mixed the wet lay-up chemicals throughout the secondary side in a somewhat shorter period of time than bubble-agitation technique alone, it has proved to be expensive and cumbersome since a substantial amount of effort is required by the maintenance personnel to install, operate and remove the pump and various hoses. It has been found that the pump recirculation technique is of such limited effectiveness due to the phenomena known as "streaming" in the art of fluidics. The practical effect of such streaming is that the jet of pressurized water created by the pump passes through the rest of the water largely intact, without mixing. While the effect of such streaming can be counteracted by the installation of a multiplicity of hoses and nozzles, the time loss associated with the installation and removal of additional hoses would more than offset any time gain realized as a result of an increased mixing rate. The pump circulation technique has the additional drawback of increasing the amount of radiation exposure of the maintenance personnel, since they must install and remove the hoses and pump.
Clearly, a new technique for the rapid and uniform mixing of anti-corrosive wet lay-up chemicals within the secondary side of a nuclear steam generator is needed that minimizes both the downtime of the steam generator and the radiation exposure of the maintenance personnel. Ideally, such a technique should be highly reliable, inexpensive, and readily applicable to all models of nuclear steam generators now in existence.